In the present paper we confirm and extend previous studies showing heterogeneous 3H-imipramine (3H-IMI) binding sites. Inhibition curves of various drugs (serotonin, imipramine, desmethyl-imipramine, d-fenfluramine, d-norfenfluramine and indalpine, a potent serotonin uptake inhibitor) obtained using 2 nM 3H-IMI and in presence of 120 mM NaCl, confirmed the presence of at least three 3H-IMI binding sites: two of these (high and low affinities) were serotonin-insensitive while the third one was selectively inhibited by serotonin and indalpine with nanomolar affinities. Moreover, this last component was found to be selectively modulated by chronic imipramine treatment thus suggesting a closer relation to serotonin uptake mechanism. These data indicate that the use of a more selective inhibitors of the serotonin-sensitive component (like indalpine or serotonin itself) to define non specific 3H-IMI, may be of help in understanding its relation with serotonin uptake system.
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